If you are using square waves, then the frequency doesn't matter at all, it could be a 1 Hz square wave and still suffer from degradation due to mismatched impedance. What matters is the highest frequency component of the square wave, which is determined by the rise/fall time of the edges, not the period (repetition frequency) of the square wave.

Geesh, always a technical stickler in the group. Yes, what you say is true for an ideal network on paper. But I was speaking from a practical standpoint to help jonnycat. Not to teach electrical engineering 101. Square wave degradition as frequency increases has mostly to do with the source generator, and maybe a little to do with the monitoring oscilloscope bandwidth.

At one time or another I have owned and used all of the better commercial bench oscillators. HP, Tek, Krohn Hite, Wavetek, Bruel & Kjaer, you name them. As you increase the frequency, the leading sharp corner rounds off and the waveshape looks less and less square the higher you go. No generator is perfect because they all have built in compromises.